1. Field of the Invention
The present invention relates to a laser irradiation apparatus used to crystallize a semiconductor film. Moreover, the present invention relates to a laser irradiation method and a method for manufacturing a semiconductor device that use the laser irradiation apparatus.
2. Related Art
A thin film transistor formed using a poly-crystalline semiconductor film (this thin film transistor is hereinafter referred to as a poly-crystalline TFT) is superior in mobility by two digits or more compared to a TFT formed using an amorphous semiconductor film. Therefore, the poly-crystalline TFT has an advantage that a pixel portion and its peripheral driver circuit of a semiconductor display device can be integrally formed over a same substrate. The poly-crystalline semiconductor film can be formed over an inexpensive glass substrate by using a laser annealing method.
In the lasers, there are a pulsed laser and a continuous wave (CW) laser according to the oscillation method. The output power per unit time of the pulsed laser typified by an excimer laser is approximately three to six digits higher than that of the CW laser. Therefore, the laser irradiation can be performed efficiently to the semiconductor film by shaping a beam spot (the region on the surface of the processing object where the laser light is irradiated in fact) of pulsed laser light into a rectangle having a length of several cm in a side or a line having a length of 100 mm or more through an optical system. That is to say, the pulsed laser has an advantage of high throughput. For this reason, the pulsed laser has been mainly used to crystallize the semiconductor film.
It is noted that the term of linear herein used does not mean a line in a strict sense but means a rectangle having a large aspect ratio (or an oblong). For example, the rectangular having an aspect ratio of 2 or more (preferable in the range of 10 to 10000) is referred to as the linear. It is noted that the linear is still included in the rectangular.
The semiconductor film crystallized thus by the pulsed laser has an aggregation of crystal grains each having a random position and a random size. Compared to the inside of the crystal grain, a boundary between the crystal grains (a crystal grain boundary) has an amorphous structure and an infinite number of recombination centers and trapping centers existing due to crystal defects. When a carrier is trapped in the trapping center, potential of the crystal grain boundary increases to become a barrier against the carrier, and therefore a transporting characteristic of the carrier is lowered.
In view of the above problem, recently, attention has been paid to a technique of crystallizing the semiconductor film using a CW laser. Unlike the conventional pulsed laser, the CW laser makes it possible to grow crystal continuously toward the scanning direction and to form an aggregation of crystal grains including a single crystal extending long in the scanning direction by irradiating the semiconductor film with the CW laser light while scanning the CW laser light in one direction. With this technique, it is considered that a semiconductor film having almost no crystal grain boundary at least that intersects the channel direction of the TFT can be formed.
The laser light having higher absorption coefficient to the semiconductor film can crystallize the semiconductor film more efficiently. In the case of crystallizing a silicon film having a thickness from several tens to several hundred nm, which is generally used in a semiconductor device, with the use of a YAG laser or a YVO4 laser, a second harmonic having a shorter wavelength than a fundamental wave is usually used because the second harmonic has much higher absorption coefficient than the fundamental wave. The fundamental wave is hardly used in the crystallization step. The harmonic can be obtained by converting the fundamental wave using a non-linear optical element.
However, the CW laser has a problem that the resistance of the non-linear optical element against the laser light is much lower than that when using the pulsed laser because the CW laser burdens the non-linear optical element continuously. Moreover, since the CW laser has lower output power than the pulsed laser per unit time, the density of photon to time is also low, and therefore the conversion efficiency into the harmonic using the non-linear optical element is low. Specifically, although the conversion efficiency depends on the mode characteristic or the time characteristic of the incident light, the CW laser generally has the conversion efficiency from approximately 0.2 to 0.3% while the pulsed laser has the conversion efficiency from approximately 10 to 30%.
Therefore, since the harmonic of the CW laser has lower power than that of the pulsed laser per unit time, it is difficult to increase the throughput by enlarging the beam spot of the CW laser. For example, the fundamental wave of the CW YAG laser can have a power of 10 kW while the second harmonic thereof can have a power as low as approximately 10 W. In this case, in order to obtain the energy density that is required to crystallize the semiconductor film, it is necessary to make the square measure of the beam spot as small as approximately 10−3 mm2. As thus described, the CW laser is inferior to the pulsed excimer laser in terms of throughput, and this causes low productivity.
It is noted that opposite ends of the beam spot in the direction perpendicular to the scanning direction form a region in which the crystal grain is extremely small and the crystallinity is inferior compared to the center of the beam spot. Even though a semiconductor element is formed in such a region, high characteristic cannot be expected. Moreover, although the adjustment of the optical system can decrease to some extent the region in which the extremely small crystal grain is formed, this region cannot be eliminated completely. Therefore, it is important to enlarge the width of the beam spot in the direction perpendicular to the scanning direction in order to relax the restriction on the layout of the semiconductor element. However, in the case of the CW laser, it is difficult to enlarge the beam spot according to the above reason, and the width of the beam spot of the CW laser is narrower than that of the pulsed laser. Therefore, the layout of the semiconductor element is restricted.
Although the CW laser can form the nonequilibrium state thermodynamically, the CW laser has an output power as low as approximately several kW in contrast with the pulsed laser having a peak power as high as several MW or more. Therefore, even in the nonequilibrium state, the CW laser is not preferable because the CW laser gives more thermal damage to the glass substrate than the pulsed laser when performing the laser annealing to the semiconductor film over the glass substrate. When the thermal damage is given too much to the glass substrate, the glass substrate shrinks accordingly.